The performance announced Thursday indicates that Google is still struggling to close the gap between the rates for ads shown on mobile devices and those on personal computers.

Advertisers haven't been willing to pay nearly as much to reach prospective customers on the smaller screens of smartphones and tablets, but Google Inc. has been tweaking its digital marketing system so mobile and PC ad campaigns are bundled together.

Google's average ad price during the fourth quarter fell 11 percent from the previous year.

The company earned $3.4 billion, or $9.90 per share. That compares with $2.9 billion, or $8.62 per share, in the prior year.